Spin doctors: Size matters

"Over 80 per cent of women say that this shampoo leaves their hair healthier and shinier." Such claims are common in advertising for all manner of consumer products. What they might not tell you is that only five women tested the shampoo. And of the four who certified its miraculous effect, one or two probably ended up with nicer hair purely by chance, or simply imagined the results.

Similar caveats apply to the effectiveness of medical treatments. Curing six out of 10 patients is promising. Curing 300 out of 500 is the same success rate, but far more convincing. "The sample size in a test is absolutely crucial in deciding whether any apparent improvement could have happened by chance alone," says Spiegelhalter.

The standard procedure for such trials is the one established by Bradford Hill over 60 years ago: new medical treatments are tested in randomised controlled trials (RCTs), in ...

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